Coatings are commonly applied to metals to make them cosmetically appealing or to protect them against deterioration during service. The metal can be formed into the shape of the finished article and then the coating can be applied. If the coating can survive the forming operation, it can be applied to the metal before forming, making it unnecessary to apply another coating on the finished article.
It is usually advantageous to apply the coating onto the metal in sheet or coil form before the forming operation. Two major advantages are that it is much easier to apply thin coatings of uniform thickness to flat metal than to an irregularly shaped formed article and that the coating provides lubrication to protect the metal during forming.
The requirements which a coating must meet depend on the particular application for which the article is intended. A major requirement for any coating which is applied prior to the forming operation is that the coating must survive the metal forming operation. Many types of coatings which would otherwise fulfill an adequate decorative or protective function fail if applied before forming because they crack during the forming operation and therefore can no longer fulfill their decorative or protective function. It is common to heat precoated metal in the forming equipment to temperatures as high as 100.degree. C. to soften the coating so it will survive forming.
There are many applications for coated metals. One example in particular is in coatings for metal containers. Metal containers that are formed by single step drawing or multiple step drawing are usually coated for lubrication. Coating of the metal is also desired for protection of the metal prior to and subsequent to formation of the container. An ideal coating would protect the metal from corrosion prior to the formation of the container, lubricate the metal during severe drawing to minimize the strain on the metal, protect the metal after formation of the container, and withstand any further processing of the container such as heating or steam sterilization. Furthermore, the coating on the inside of the container must be compatible with the contents of the container and the coating on the outside of the container should be resistant to scuffing for the life of the container.
U.S. Pat. No. 3,792,005 disclosed that coatings can be made using block copolymers of the A-B-A type where A is a monovinyl aromatic polymer block, usually polystyrene (S), and B is a rubber polymer block, usually hydrogenated polybutadiene (EB) or hydrogenated polyisoprene (EP). These polymers could be especially useful in metal coatings applied prior to forming because they can be formulated to have good flexibility and therefore, will not crack during metal forming. However, coatings based on conventional A-B-A type block copolymers are deficient in that they lack strong adhesion and in applications in which the coating will contact organic liquids such as gasoline, the coatings will merely dissolve off of the metal. U.S. Pat. No. 3,970,771 disclosed a primer composition to promote adhesion of block copolymer based coatings to metals.
S-B-S type block copolymers, wherein the B block is an unsaturated polybutadiene, have been mixed with oils to prepare removable thermoplastic rubber coatings for metal that is subjected to mechanical deformation as disclosed in U.S. Pat. No. 3,923,673. The S-B-S block copolymer coating is quite flexible and withstands deformation of the metal while providing lubrication between the metal and the metal deforming equipment. However, the thermoplastic rubber and oil coatings are not ideally suited for the container industry since the coating is removable and would require the application of a final coating.
Chlorinated natural and synthetic rubbers have been grafted with acrylic acid compounds to form coatings for metals as disclosed by U.S. Pat. No. 4,439,587. High acrylic acid content is disclosed as allowing aqueous dispersions of the coating which can be crosslinked with aminoplast resins.
Metal container coatings have also been formed from a latex of a high nitrile polymer made by emulsion polymerization of acrylonitriles, styrenes, and dienes as disclosed in U.S. Pat. No. 4,329,401. The latex can be applied to the metal by rolling, spraying, dipping and other means.